This function does the same thing as
getgrgid
(3): it looks
up a group file entry by group number.
The return value in list context is:

($name, $passwd, $gid, $members)

where
$members
contains a space-separated list of the login names of the members of the group.
If you want to do this repeatedly, consider caching the data in a hash
(associative array) using
getgrent
.